Improved support for W=2 compilation flag by solving some nested-extern
and sign-compare warnings.

The libraries are compiling with warnings (which turn into errors with
the Werror flag).

Outside of libraries, some warnings cannot be fixed.

Change-Id: I06b1923857f2a6a50e93d62d0274915b268cef05
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>

Change-Id: I8989d2aa0258bf3b50a856c5b81532d578600124
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>